Generally speaking, a steering apparatus of a vehicle is to assist changing the direction of vehicle as the driver intends by turning front wheels in the same direction about their respective pivot axes.
Meanwhile, the hydraulic power steering apparatus assists a driver's operation of the vehicular steering wheel with a boosting device for boosting a steering force to change directions of the vehicle.
Such hydraulic power steering apparatuses may be generally classified into HPS (Hydraulic Power Steering Apparatus) and EPS (Electric Power Steering Apparatus). In the hydraulic power steering apparatus, an oil pump connected to an engine rotary shaft supplies an actuation fluid to an actuation cylinder connected with a rack bar to allow the driver to steer with the boosted force.
In contrast, instead of the oil pump and actuation cylinder, the electric power steering apparatus has a rack bar or a column-installed motor and an ECU (Electronic Control Unit) for a motorized operation of the steering maneuver. Although recent trends show increased use of the electric power steering apparatuses, most vehicles still carry hydraulic power steering apparatuses.
FIG. 1 is a perspective view of a vehicle hydraulic power steering apparatus 100 and FIG. 2 is a sectional view of the same. As shown, the hydraulic power steering apparatus 100 is installed at the lower end of a steering shaft (not shown) to which a steering wheel (not shown) is mounted and comprises a pinion valve assembly 120 for controlling the direction of the actuation fluid in steering and an actuation cylinder 140 for applying an actuation force to tie rods 115, which steer the vehicle wheels (not shown) in response to the operation of the pinion valve assembly 120.
The hydraulic power steering apparatus 100 comprises a hydraulic system where the actuation fluid supplied from oil pump 130 may be delivered to pinion valve assembly 120, selectively fed to two pressure chambers 205, 210 at opposing sides about a piston inside actuation cylinder 140 in response to the rotational directions of the steering shaft to assist the driver's steering maneuver, and then retires its actuations from pinion valve assembly 120 by flowing back to an oil reservoir tank 131 and finally supplied back to oil pump 103 via an inlet hose 132.
FIG. 3A is a sectional view of a pinion valve assembly of a conventional vehicle hydraulic power steering apparatus and FIG. 3B is a sectional view of a conventional pinion valve body.
As shown in FIG. 3A, a pinion valve assembly 120 has a hollow input shaft 320 rotating in association with a steering wheel (not shown), a torsion bar 310 inserted in the interior of input shaft 320 and fastened at its top to input shaft 320 by a pin 315, a pinion shaft 350 press-fitted with a lower end of torsion bar 310 and meshed with a rack gear of a rack bar 355 of an actuation cylinder and a pinion valve body 340 fastened to an upper end of pinion shaft 350 by a pin 347 and sleeved over input shaft 320.
The thus constructed hydraulic power steering apparatus boosts a manual steering force, which rotates a steering shaft (not shown) along with input shaft 320 of pinion valve assembly 120, when actuation fluid, which is under pressure from an oil pump (not shown) and supplied through a supply port 345, is selectively fed into a left or right pressure chamber (see chambers 205, 210 of FIG. 2) of the actuation cylinder via a left port 335a or a right port 335b. 
As shown in FIG. 3B, pinion valve body 340 of the conventional hydraulic power steering apparatus is made by lathe-turning a hollow cylindrical steel member to machine its interior and exterior surfaces followed by broaching or slotting to form a pin hole 360, O-ring grooves 380, a supply hole 311, a left discharge hole 313, a right discharge hole 315, etc. Then, pinion valve body 340 needed press-fit assembly of top and bottom sleeves 370 for preventing oil leaks resulting in a higher material cost as well as an extra manufacturing cost due to the lathe-turning, broaching or slotting to add to the manufacturing process and total number of components.